1. Field of the Invention
This invention relates to energetic inorganic salts and more particularly to salts containing the NF.sub.4.sup.+ cation.
2. Description of the Prior Art
NF.sub.4.sup.+ salts are key ingredients for solid propellant NF.sub.3 -F.sub.2 gas generators, as shown by D. Pilipocich in U.S. Pat. No. 3,963,542, and for high detonation pressure explosives, as shown by K. O. Christe in U.S. Pat. No. 4,207,124. The synthesis of NF.sub.4.sup.+ salts is unusually difficult because the parent molecule NF.sub.5 does not exist and the salts must be prepared from NF.sub.3 which amounts formally to a transfer of F.sup.+ to NF.sub.3 accordingly to: EQU NF.sub.3 +F.sup.+ .fwdarw.NF.sub.4.sup.+
Since fluorine is the most electronegative of all elements, F.sup.+ cannot be generated by chemical means. This difficult synthetic problem was overcome by K. O. Christe and co-workers, as shown in U.S. Pat. No. 3,503,719. By the use of an activation energy source and a strong volatile Lewis acid, such as AsF.sub.5, the conversion of NF.sub.3 and F.sub.2 to an NF.sub.4.sup.+ salt became possible: ##STR1## However, only few Lewis acids are known which possess sufficient strength and acidity to be effective in this reaction. Therefore, other indirect methods were needed which allowed conversion of the readily accessible NF.sub.4.sup.+ salts into other new salts. Two such methods are presently known. The first one involves the displacement of a weaker Lewis acid by a stronger Lewis acid, as shown by K. O. Christe and C. J. Schack in U.S. Pat. No. 4,172,881 for the system: EQU NF.sub.4 BF.sub.4 +PF.sub.5 .fwdarw.NF.sub.4 PF.sub.6 +BF.sub.3
but obviously is again limited to strong Lewis acids. The second method is based on metathesis, i.e., taking advantages of the different solubilities of NF.sub.4.sup.+ salts in solvents such as HF or BrF.sub.5. For example, NF.sub.4 SbF.sub.6 can be converted to NF.sub.4 BF.sub.4 according to: ##STR2## This method has successfully been applied by K. O. Christe and coworkers, as shown in U.S. Pat. Nos. 4,108,965; 4,152,406; and 4,172,884, to the syntheses of several new salts. However, this method is limited to salts which have the necessary solubilities and are stable in the required solvent. The limitations of the above two methods are quite obvious and preempted the syntheses of NF.sub.4.sup.+ salts of anions which are either insoluble in those solvents or are derived from a Lewis acid weaker than the solvent itself and therefore are displaced from their salts by the solvent.